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"Yes" = "hai" and is pronounced like the English word "hi." "No" = "iie" and is pronounced with "ii" like the long vowel sound in "meek" and "e" like the short vowel sound in "bet." Japanese is a phonetic language so you should be able to just say the words as they're "spelled."

There are two ways to use "four" and "seven" in the Japanese language. Basically, when you are counting (like from one to ten), you can use either way. If you are using it as a number outside of the single digits place like "400" or "70," you would use "yon" for four and "nana" for seven (and not "shi" and "shichi"). It's a lot more complex than this, really.

"I don't know" is "wakarimasen."

If you're learning all of this for aikido, I wouldn't really bother. You're not there to learn the Japanese language but to learn aikido. Learning the technical terms, I think, is important, but all of the rest of the language, I believe, would be superfluous. I doubt that any dojo would force you to learn all of these kinds of Japanese trappings. If they do, I would be a bit wary...